Stock buy-back raises the price of still-held shares in two ways: first, it creates market demand for the firm's shares. Second, it reduces supply of said shares. Increasing demand and reducing supply are both standard micro-economic ways to increase prices.
As Meyerson notes, stocks are being traded more frequently than was once the case. Profits are thus harvested via capital gains, taxed at a lower rate than dividend income.
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I suggest another possible cause for wage stagnation: skyrocketing healthcare costs. It is likely that money is being diverted from take-home pay to healthcare premiums. I'd like to see whether total cost-to-employ has risen, while wages have not, I predict it could be a factor.